Regenerated cellulose structures and method for preparing same



Patented May 3, 1938 REGENERATED CELLULOSE STRUCTURES AND METHOD FORPREPARING SAME Edward R. McKee, Hermitage, Tenn, assignor to E. I. duPont de Nemours & Company, Wilmington, Del., a corporation of DelawareNo Drawing. Application October" 21, 1936,

Serial No. 106,821

4 Claims.

This invention relates to the manufacture of regenerated cellulosestructures, such as yarns,

filaments,-threads, horsehair, straw, film, caps.

bands;tubes and the like. More particularly, this invention relates tothe process of purifying regenerated cellulose structures produced fromviscose and especially to that step in the purification procedure knownas bleaching.

In the manufacture of regenerated cellulose structures from viscose, thefreshly formed structures contain large quantities of impurities andreaction products, most of which are objectionable and must be removed.The usual purification treatments comprise washing the freshly producedregenerated cellulose structures to remove the acid and other watersoluble compounds, then treating the structures with a desulfuringsolution to remove,the sulfur and sulfur compounds and finally bleachingto remove coloring matter and the like which have resisted removal inthe prior treatments.

Complete and uniform removal of the impurities and reaction productsrequires careful manipulation because the regenerated cellulosestructures are easily degraded by too severe chemical'treatments, or byrepeated handling or mechanical treatments, especially when anartificial silk thread constitutes the cellulosic structure. Generally,the purifying treatment of thread is carried out in the form of skeinswhich is not only a long, tedious operation, but results in degradationof the yarn both from mechanical handling and severe chemical treatment.such as bleaching.

I1. application Serial No. 683,896 to Fonda and Filson, filed August 5,1933, there is disclosed a package bleaching process which comprisestreating cakes of regenerated cellulose thread prepared by the viscoseprocess with chlorinated water which contains, for instance. from 3--100parts per million of chlorine and which is substantially neutral, havinga pH between 6.0 and 8.0. This process is a decided improvement overearlier prior art processes for the very dilute solution of chlorine inwater eflec'tively bleaches reenerated cellulose structures and in thecase of hard, compact packages of yarn, such as may be produced bycollecting freshly formed thread incentrifugal spinning buckets rotatingat high speeds,'good penetration and uniform bleaching isobtainedbecause the solution does not swell the thread which would tendto close up the interstices between the thread windings.

It has been found, however,. that when rayon packages are bleached'inaccordance with the Fonda and Filson application just previouslyreferred to and dried in the usual manner, the yarn layers on theoutside of the cake tend to darken due to a trace of hydrogen sulfide inthe air that is circulated through'the drier and which probably reactswith a trace of lead or lead compounds contained in or on the yarn.

It is therefore an object of this invention to provide a modifiedchlorinated water bleaching solution which will not only eiiectiveiybleach compact packages of rayon, but will also eliminatesubstantialdarkening of the outer layers of the thread package after the yarn isdried in the usual manner. Other objects of the invention will appearhereinafter.

The objects of the invention may be accomplished, in general,by'treating the regenerated 'cellulose structures to be bleached with ableachingsolution containing 3-100 parts of.chlorine per million partsof water and which has a pH value of between 2.0 and 4.0.

The bleaching solution of the present invention has substantially thesame chlorine concentration as that disclosed in the above-mentionedFonda and Filson application. The latter bleaching solution ismaintained substantially neutral, having a pH value of between 6.0 and8.0, whereas the bleaching solution of this invention is decidedlyacidic having a-pH value of between 2.0 and 4.0. The pH value of thebleaching solution will be obtained by the addition of an acid,preferably a strong mineral acid such as sulfuric acid, nitric acid,hydrochloric acid, phosphoric acid or the like. Obviously the amount ofany acid to be added to obtain the desired pH value can be readilydetermined after ascertaining the alkalinity of the water. The use of ableaching solution having a pH value of 2.0 to 4.0 will su :tantlallyprevent the formation of dark stains on the outer surface of a yarn cakeor other regenerated cellulose structure without deleteriously affectingthe physical properties of the yarn.

In order to more fully explain the nature of the invention, thefollowing examples which are illustrative and in no waylimitative of theinvention are given.-

E'mampie I I A relatively large dense package such as a freshly spun.acid cake of viscose'rayon is collected in-a bucket revolving at about8,000 R. P. M., the bucket being the type disclosed in the copendingapplication of S. W. Brainard, Serial No. 628,317. The cake so producedwhich con- 7 tains about 0.8 pounds of 150 denier-40 filament yarn isdrip washed for 8-10 hours with soft water at. approximately 25 C.Thereafter, the cake is subjected to a desulfuring treatment whichcomprises drip washing with sodium carbonate solution at 00-100 C. forapproximately 3-5 hours. The cake is then again washed with soft waterby the drip method for 1 hour, after which the cake is treated or washedby the drip method for 10-14 hours with chlorinated water containing 20parts per million of chlorine (0.002% a'nd having a pH value of 3.0 dueto the addition of the required amount of sulfuric acid to the waterprior to the chlorination thereof. The cake is again washed with softwater, using the drip method for 1 hour, after which it may be finished.

, A bobbin of viscose yarn which has been washed acid free, desulfuredand dried to less than 20% moisture content is rewound with twistingupon a perforated bobbin in the manner set forth in U. S. Patent No.2,004,153. This yarn is then subjected to a pressure washing treatmentfor 5-6 hours at 5-10 pounds pressure. depending upon the permeabilityof the package, (the flow through the package being about 6-10 gallonsper hour). Soft water which has been acidulated slightly with nitricacid and then chlorinated to the extent of -40 parts per million ofchlorine (0.003%-0.004% available chlorine) and with a final pH value of2.0 to 4.0 is used at a temperature of approximately 30 C. At the end ofthe treatment, the bobbin of yarn is separately washed in chlorine freewater for 15 minutes and then dried. Such bobbins of yarn may be wounddirectly into cones to be sold as bleached yarn.

Example III A freshly formed bucket cake of regenerated cellulose threadspun from viscose and collected in a normal or round centrifugalspinning bucket rotating at about 8,000 R. P. M. until a cake of about0.8 pound is formed is wrapped in cheesecloth and then processed in themanner described in Example I. The solution contains sumcient nitricacid to bring the pH of the solution to between 3.0 and 4.0.. The cakeso treated is uniformly bleached and shows no dark-stains on thesurface.

It is obvious, of course, that other methods of applying the treatingsolution to the yarn packages may be used. such' as for instance, vacuumnot appreciably swell the yarn, behaving in thisrespect the same as asubstantially neutral solution. Thus, gel yarn, that is, yarn which hasnever been dried to equilibrium and which is more reactive than yarnwhich has once been dried and which is readily and largely swelled bythe usual bleaching solutions which are alkaline, may be bleachedsatisfactorily in package form with solutions of chlorinated waterhaving a pH from 2.0 to 4.0.

A pH of from 2.0 to 4.0 is preferred because the activity of the bleachis substantially as high as with a neutral solution. In scme cases asomewhat lower pH may be used to more readily effect a removal of theimpurities in the yarn, in which case a higher concentration of chlorineis advisable to offset the lowering'of bleaching activity.

Though the invention in its preferred embodiment contemplates.treatingdense-packages of gel rayon produced by the viscose process, itis obvious that the invention is applicable to regenerated cellulosethreads prepared from viscose in anyform and to other regeneratedcellulose structures Obviously many changes and modifications canbe madein the above described procedure with-- out departing from the natureand spirit of the invention. It is therefore to be understood that theinvention is not to be limited ekcept as set forth in the appendedclaims.

What is claimed is:

1. A bleaching solution suitable for use in the bleaching of regeneratedcellulose structures comprising a solution containing between 3'and 100parts chlorine per million parts water and having-a pH value of 2.0 to4.0.

2. The method of bleaching regenerated'cellulose structures whichcomprises treating said structures 'with an aqueous solution of.chlorine having a pH value of less than 4.0.

3. The method of bleaching regenerated cellulose structures whichcomprises treating said structures with a solution containing between 3and 100 parts chlorine per million parts water and having a pHjvalue of2.0 to 4.0.

4. A bleaching solution suitable'for use in the bleaching of regeneratedcellulose structures com- EDWARD R. MCKEE.

